This invention relates to apparatus for supplying natural gas fuel for the purpose of heating the boilers of tankers that transport liquefied natural gas (LNG).
An LNG tanker is conventionally propelled by one or more steam turbines. The tanker is thus provided with boilers to raise the steam. About 50% of the fuel that is required to be burnt in order to raise the steam provided from the LNG storage tanks themselves of the tanker. Since liquid natural gas has a boiling point at a temperature of about minus 162xc2x0 C., even though the storage tanks are thermally insulated, the boiling point of liquid natural gas is so low that there is inevitably a continuous boil-off of LNG from the tankers, albeit at a modest rate. Conventional operation is to employ such natural boil-off to meet a part of the requirements for fuel of the boilers. The balance is met from a stock of a heavy hydrocarbon liquid fuel, sometimes referred to as xe2x80x9cbunker oilxe2x80x9d.
It is however required that the tanker should have the capability of generating sufficient thermal energy from the combustion of the natural gas alone to raise all the steam required for its propulsion. Accordingly, the apparatus for supplying natural gas to the burners associated with the boilers additionally includes a forced vaporiser. In the tanker apparatus that has been used for the past 20 years there is a natural gas compressor which receives part of its feed from the ullage space of the LNG storage tanks and the remainder from the forced vaporiser. The forced vaporiser has an outlet temperature in the order of minus 40xc2x0 C., chosen to be well above the dew point of the natural gas so as to ensure that there is no precipitation of liquid droplets in the inlet to the compressor. On the other hand the temperature of the natural gas taken from the ullage space of the tanks tends to be in the range of minus 100xc2x0 C. to minus 140xc2x0 C., the latter temperature obtaining when the tanker is fully laden with LNG, the former temperature obtaining when the tanker returns from making a delivery, a small amount of LNG having been retained for purposes of keeping the storage tanks at a suitable low temperature. In practice, therefore in this conventional apparatus the compressor has to be arranged so that it can operate at any natural gas inlet temperature in the range of minus 40xc2x0 C. to minus 140xc2x0 C. At the lower temperature the adiabatic work required to compress the gas is in the order of 20 to 25 kJ/kg whereas at the higher temperature it is in the order of 50 to 60 kJ/kg. The motor which drives the compressor therefore has to be able to cope with a wide range of duties. Typically, the motor is either provided with a frequency converter or is of a plural speed kind. Other disadvantages arise. In particular, it is necessary to employ a mist separator downstream of where the gas from the forced vaporiser is mixed with the natural boil-off gas so as to ensure that no particles of liquid are carried into the compressor, such particles being a potential source of liquid erosion damage to the impeller of the compressor.
There is therefore a need for an improved arrangement for providing natural gas as a fuel to raise steam in the boilers of the tanker and the invention aims at providing an apparatus that meets this need.
According to the present invention there is provided apparatus for supplying natural gas fuel to heat the boilers of an ocean-going tanker for the transport of LNG comprising a compressor having an inlet communicating with the ullage space of at least one LNG storage tank and an outlet communicating with a conduit leading from the compressor to fuel burners associated with the boilers, and a forced LNG vaporiser having an inlet communicating with a liquid storage region of the said tank and an outlet communicating with the same or a different conduit leading to fuel burners associated with the boilers.
The invention makes it possible to operate the compressor within a narrower temperature range, typically minus 140xc2x0 C. to minus 100xc2x0 C., thereby reducing greatly the amount of work required to compress the natural gas to a given pressure. As a result, a single speed motor without a frequency converter can typically be used to drive the compressor. Accordingly, the compressor drive arrangement is simplified. In addition, the power consumed by the compressor is reduced. There are two reasons for this. First, its average operating temperature is lower than in the known apparatus described above. Accordingly, the work needed to raise a unit quantity of this gas by a chosen pressure is less than in the known apparatus. Second, because the forced vaporiser exhausts downstream of the compressor, the rate of gas flow through the compressor is reduced in comparison with the known apparatus.
Preferably, the forced vaporiser has a second valve means to control the flow associated therewith, a first valve means operable to select its outlet temperature and a second valve means to control the flow rate of gas therethrough.
Typically, there is a gas heater downstream of the compressor. If the forced gas vaporiser communicates with the same conduit as the outlet from the compressor, the location in this conduit at which the forced vaporised gas encounters the compressed gas is preferably downstream of the gas heater.